The present invention relates to a current-limiting device for high voltage switching mechanisms having a fast acting circuit breaker for limiting and interrupting short-circuit currents. In the circuit breaker, an explosive charge is ignited to break an explosive conductor and the resulting arc voltage commutates the current to be limited to a fusible conductor connected in parallel with the explosive conductor wherein the current is quickly interrupted and thereby limited.
Fast acting circuit breakers of the type described which can be used as current-limiting device are disclosed, for example, in West German Pat. Nos. 1,056,249 and 1,169,019. They are suitable for use both with direct and alternating currents. Their switching times generally range from ten to one hundred times less than the switching times of other circuit breakers of conventional design. The time for extinguishing the current in the event of a short-circuit is typically less than 0.5 msec. It is thereby possible to keep the short-circuit current so small in alternating current power lines of conventional frequency that the electrical switching mechanisms and associated equipment no longer need to be rated for the full surge current (I.sub.s). In view of its current-limiting effect, the described apparatus is often denoted in technical literature as an "I.sub.s limiter". A discussion of the multiple applications of I.sub.s limiters can be found, for example, in the paper entitled "Selective Switching in Extremely Short Times Using I.sub.s Limiters" located in ETZ-A, Vol. 81, No. 20/21 of Sept. 26, 1960, at pp. 741-744.
I.sub.s limiters basically comprise switches which are commonly used as circuit breakers. In power supply networks, it sometimes becomes necessary to reestablish the original connection of the power supply circuit after a successful opening of the circuit as a result of a short-circuit. If the short-circuit continues, as a rule, the final circuit opening must then take place. For this reason, switching mechanisms in high voltage networks are provided with devices for quick reconnection into the circuit. The quick reconnection into the circuit takes place through circuit breakers which are specially designed for this purpose.
An object of the present invention is to provide a curent-limiting device which is capable of using fast acting circuit breakers, such as I.sub.s limiters, to carry out quick reconnections into a power supply circuit, and preferably to provide entire quick reconnection cycles.
In order to achieve this objective in accordance with the present invention, at least one additional fast acting circuit breaker and a reconnect switch connected in series therewith are connected in parallel with the primary fast acting circuit breaker. In addition a device for establishing the readiness of the additional circuit breaker to explode or trip in response to a short-circuit can be connected in parallel with the additional circuit breaker.
The advantages of the present invention include the following points, among others: (a) extremely rapid response to short-circuits since the fast acting circuit breakers require typically less than 0.5 msec to limit and extinguish the short-circuit current; (b) the short-circuit current in alternating-current power lines can then be kept so small that electrical switching mechanisms and associated equipment no longer need to be rated for the full surge current. This is particularly of great economic significance in the construction of high voltage networks where increases in the breaking capacity of the circuit breakers are not required. In extreme cases, the exchange of all circuit breakers of a high voltage system would normally be necessary, but can be avoided by use of the current-limiting device of the present invention; and (c) by parallel connection of a number of fast acting circuit breakers with associated reconnect switches, quick reconnection in circuit cycles can be produced as desired.
Preferably, SF.sub.6 safety grounding switches are used as short-circuiting devices. Such switches are described in the company document CH-A 352 336 D of the assignee of the present invention, BBC Aktiengesellschaft Brown, Boveri & Cie., Baden, Switzerland.
According to a further development of the present invention, the device for establishing the readiness of the additional circuit breaker for exploding or tripping includes auxiliary contacts on the reconnect switch which serve to activate the ignition device of the circuit breaker when closed. These auxiliary contacts close the tripping circuit of the ignition devices along with the actuation of the reconnect switch. A breaking of the explosive conductor accordingly takes place with the actuation of the reconnect switch when a corresponding tripping signal is simultaneously available through the auxiliary contacts. This feature offers the further advantage that no measures have to be undertaken to disconnect the auxiliary and main contacts of the reconnect switch from a source of potential when exchanging circuit breakers. The same holds true for the ignition devices of the circuit breakers, which are conventionally connected to the high voltage potential, due to economic reasons.
The ignition devices can be supplied with power by means of a pulse transformer. The pulse transformer can be connected to a short-circuit supervisory control device. The pulse transformer isolates the tripping circuit from the short-circuit detection circuit to inhibit misoperation.
Fast acting circuit breakers are manufactured at the present time for rated voltages up to approximately 36 kV. At higher rated voltages, a number of circuit breakers are connected in series, or a series connection of a number of explosive conductors can be connected in parallel with a fusible conductor. Circuit breakers of this type specified for higher rated voltages are described in the West German Pat. No. 1,141,015. Both of these types of circuit breakers can be used in the current-limiting device of the present invention having the capability for quick reconnection in circuit.
The number of switching units consisting of circuit breakers and reconnect switches connected in parallel with the first circuit breaker is determined in accordance with the required switching cycles. For a simple quick reconnection in circuit (ON-OFF-ON, or O-CO according to the IEC), it is sufficient to have a device with a first circuit breaker having a series connected circuit breaker and reconnect switch connected in parallel thereto. For each further switching cycle ON-OFF (C)), an additional switching unit consisting of a series connected circuit breaker and reconnect switch is required.
It is desirable to exchange "ignited" circuit breakers with unused circuit breakers during operation of the high voltage system. For this purpose, and further in accordance with the present invention, the circuit breakers or their associated reconnect switches are connected to the high voltage busbars by means of plug contact arrangements.
The invention is further described in detail below in conjunction with the accompanying drawings.